1. Field of the Invention
This invention relates generally to liquid metal reactors and more particularly to a shutdown system for sodium cooled reactors.
2. Description of Related Art
The liquid metal fast breeder reactor (LMFBR) operates on the uranium-plutonium fuel cycle or thorium-U233 fuel cycle. The reactor is fueled with bred-isotopes of plutonium in the core, and the blanket is natural or depleted uranium. In theory, the number of fission neutrons emitted per neutron absorbed by Pu239, increases monotonically with increasing neutron energy for energies above about 100 keV. That means that the breeding ratio and breeding gain increase with the average energy of the neutrons inducing fission in the system. Therefore, every effort must be made to prevent the fission neutrons in a fast reactor from slowing down. This means the light-weight nuclei must largely be excluded from the core. The LMFBR has no moderator, so the core and blanket contain only fuel rods and coolant.
At the present, sodium is the chosen coolant for the modern LMFBR with an atomic weight of 23, sodium does not appreciably slow down neutrons by elastic scattering. Since sodium is an excellent heat transfer material, an LMFBR can be operated at high power density. This, in turn, means that the LMFBR core can be comparatively small. Furthermore, because sodium has a very high boiling point, the reactor core loops can be operated at high temperature and at essentially atmospheric pressure without boiling, and no heavy pressure vessel is required. The high coolant temperature also leads to high temperature, high pressure steam, and to high plant efficiency. Finally, sodium, unlike water, is not corrosive to many structural materials. The reactor components immersed in liquid sodium for years appear like new after the excess sodium has been washed off.
However, sodium has some undesirable properties. Its melting point is much higher than room temperature, so the entire cooling system must be heated before startup. This is accomplished by winding a spiral of insulated heating wire along the coolant piping, valves, and the rest of the system. Sodium is also highly chemical reactive. Hot sodium reacts violently with water and catches fire when it comes in contact with air, emitting dense clouds of white sodium peroxide smoke.
Unfortunately, sodium absorbs neutrons, even fast neutrons, leading to the formation of the beta-gamma emitter Na24, with a half-life of 15 hours. Sodium, which passes through the reactor core, therefore becomes radioactive. LMFBR plants operate on the steam cycle, that is, the heat from the reactor is ultimately utilized to produce steam in steam generators. However, because of the radioactivity of the sodium and because sodium reacts so violently with water, it is not considered a sound engineering practice to carry the sodium directly from the reactor to the steam generators. Leaks have often occurred in steam generators between the sodium on one side and the water on the other, and such leaks could lead to the release of radioactivity.
Therefore, all LMFBRs have two sodium systems: the primary system cooling the core and carrying radioactive sodium, and an intermediate system with a sodium-to-sodium intermediate heat exchanger before going to the steam generator. Thus, a hypothetical sodium water reactor will involve non-radioactive sodium. The physical arrangement of LMFBRs can be divided into two categories: the loop-type LMFBR and the pool-type LMFBR. Issues concerning the operation of the core to which this invention relates for all practical purposes are the same for both types of LMFBRs. The loop-type is a more familiar design, because except for the presence of the intermediate loop, it is not much different in design from an ordinary pressurized water reactor. All primary loop components, the reactor, pumps, heat exchangers, etc. are separate and independent. In a pool reactor all the primary system components are immersed in the primary vessel. This makes inspection, maintenance, and repairs more complicated as these components are immersed in hot, radioactive, and opaque sodium. However, the shielding requirements of a pool reactor are reduced.
Furthermore, the usual practice is to locate pool-type reactor vessels at least partially underground, so that only the upper-most portion of the vessel requires heavy shielding. It is possible to walk into the reactor room where a pool-type reactor is operating and even walk across the top of the reactor without receiving a significant radiation dose. Therefore, the pool-type LMFBR can be very tight and compact.
As an example, a pool-type of LMFBR 10 is illustrated in FIG. 1. A reactor core 16, which is the heat generation source, is supported within a pool of sodium 12 that is maintained within a vessel 14 under an inert cover gas 26 which is sealed by the vessel cover 15. A primary coolant pump 18 is suspended from the vessel cover 15 and extends into the sodium pool 12 with an intake 20 at its lower end. The sodium intake is driven through the core inlet piping 22 to an inlet plenum 23 below the core 16 from which it enters the core 16 and is heated to temperatures in the order of 930° F. (500° C.). The radioactive heated sodium then exits the core 16 through an upper plenum which directs the heated sodium through core outlet piping 24 and into the primary side of intermediate heat exchanger 28 where it is placed in heat exchange relationship with sodium passing through intermediate loop piping 32 which is driven by the pump 30. The sodium in the intermediate loop piping 32 is then conveyed to a secondary heat exchanger 34 which places the sodium in heat exchange relationship with water to generate high pressure steam which is conveyed through the steam piping 36 to drive turbine 38. The condensate is then returned to the heat exchanger 34 to close the cycle. The turbine 38 can then be used to drive a generator 40 for the production of electricity 42.
A core map of the core 16 shown in FIG. 1 is provided in FIG. 2. The core comprises an array of fuel assemblies which are hexagonal stainless steel cans, for example, that are ten to fifteen centimeters across and three or four meters long that contain the fuel and fertile material in form of long pins. Typically, an assembly for the central region of the reactor contains fuel pins at its center and blanket pins at its periphery. Assemblies for the outer part of the reactor contain only blanket pins. When these assemblies are placed together, the effect is to create a central cylindrical driver surrounded on all sides by the blanket.
The fuel pins, for example, are stainless steel tubes six or seven millimeters in diameter, containing pellets composed of the mixture of oxides of plutonium and uranium. The equivalent enrichment of the fuel, that is plutonium, range between 15-35% depending on the reactor in question. The fuel pins are kept apart by spaces or in some cases by wire wound helically along each pin. The pins in the blanket, which contain only uranium dioxide are comparatively larger in diameter, for example, about 1.5 centimeters, because they require less cooling than the fuel pins. Both fuel and blanket pins are more tightly packed in an LMFBR than in a water cooled reactor because the heat transfer properties of sodium are so much better than those of water. As mentioned above, the liquid sodium coolant enters through holes near the bottom of each assembly, passes upward through the pins, removing heat as it goes, and then exits at the top of the core.
For safety LMFBR are provided with a (primary) control system which is also able to obtain reactor shutdown, and a (secondary) system with a dedicated shutdown function. The two systems employ independent and diverse means to attain reactor shutdown. In addition, sodium cooled reactors can have excellent intrinsic safety because of strongly negative reactivity coefficients. If properly designed, the reactivity coefficients can bring the reactor to a hot shutdown (criticality) even if both control/shutdown systems are inoperative (this event is called ATWS (Anticipated Transient Without Scram)). Even though ATWS are theoretically Beyond Design Basis Accidents, they are actually factored into the design in current practice. Typical design requirements for ATWS events are: no significant fuel failures, high margin to sodium boiling, and long-term structural temperatures maintained below the ASME Level D primary system boundary limit (700° C., 1,300° F.). As mentioned, current sodium reactors have two control systems (primary and secondary) of diverse design and failure of both is considered an acceptable risk, i.e., the probability of shutdown failure is less than 10−7 per demand. However, the temperature increase during a hypothetical ATWS is included in the transient design, i.e., the reactor power is such that limiting conditions are not exceeded during an ATWS. This results in the imposition of a large design margin and it lowers the rated power of the reactor. Accordingly, the key object of this invention to eliminate the need to consider an ATWS in setting up the thermal power limits of the reactor, resulting in an economically competitive design. The threshold limit for negation of the ATWS is 10−8 events/yr. which is the value assumed for failure of the reactor vessel in LWRs, an event which is not considered in the design of LWRs.